1. Technical Field
The present invention generally relates to a method and system of measuring the axis angle of a toric lens prior to removing the lens from its mold.
2. Description of Related Art
Contact lenses having a toric optical zone (commonly referred to as “tonic contact lenses”) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for the astigmatism. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. Currently, both back toric lenses (having the tonic surface formed in the posterior lens surface) and front toric lenses (having the toric surface formed in the anterior lens surface) are available.
Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have a ballast to inhibit rotation of the lens on the eye so that the cylindrical axis of the tonic zone remains generally aligned with the axis of the astigmatism. For example, a section of the lens periphery may be thicker (or thinner) than another section to provide the ballast. Tonic contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast. This relationship is expressed as the number of degrees (rotational angle) that the cylindrical axis is offset from the orientation axis of the ballast.
Toric contact lenses, similar to spherical contact lenses, are usually offered in several different base curves, a lens parameter related to fitting characteristics of the lens, and several different spherical powers. Toric contact lenses, however, not only include the toric optical zone and ballast, but the lenses are offered with a range of cylindrical axis orientations in order to accommodate patients with different astigmatic conditions; for example, for a given base curve and spherical power, the cylindrical axis may be offered in 5 or 10 degree increments ranging from 0° to 180°. Typically, a toric lens will be thicker on the bottom, i.e., 180 degrees, to create a “ballast” to aid in self-alignment when the eye blinks. Thus, when manufacturing tonic lenses, the axis angle must be accurately measured to ensure that the lens has the proper shape to correct any given patient's astigmatism.
A toric lens prescription defines the axial offset (expressed in degrees) between the toric axis and the ballast axis of the posterior (touching the eye) and anterior surfaces of the lens, respectively. Different toric prescriptions thus have different axial offsets.
Typically the rotational offset between the ballast and toric features of anterior and posterior mold sections used to form such a lens may be selected by a programmer or operator of an apparatus housing such molds and passed through a complete production cycle to form toric lenses of any desired rotational offset. Such machinery is typically connected to a computer which is programmed to control the operation of the machine. The operator may choose and input the desired rotational offset between the anterior and posterior mold sections which is then transmitted to the appropriate machine parts which control the rotational alignment of the mold sections. The anterior and posterior mold sections may be delivered to the machine through a pair of tubes which are vertically oriented with respect to the apparatus or on a pallet. A receiving plate may be positioned directly beneath the vertically oriented tubes or adjacent to the pallet and can be configured to receive a posterior and anterior mold sections.
The receiving plate may then deposit the posterior mold section at a predetermined position within the apparatus. A posterior mold deposit rod is lowered over the posterior mold section and can lift it upward. The deposit plate may then deposit the anterior mold section onto the top end surface of the anterior mold deposit rod, with the anterior and posterior rods being axial aligned. An axis alignment tool may then be employed to a location between the posterior and anterior mold sections. In this configuration the posterior mold-deposit rod will have been lowered to engage the posterior mold section with the upper-most portion of the axis alignment tool and the anterior mold-deposit rod will have risen vertically until the anterior mold section is engaged with the lower-most portion of the axis alignment tool. Now the posterior and anterior rods may be rotated about their common vertical axis to achieve the desired toric and ballast features for the lens.
A particular quantity of liquid monomer may then be injected into the anterior mold section, and the posterior mold may be moved toward the anterior mold until they engage with a predetermined pressure. The mold sections can then be relocated for curing of the monomer to form the lens. Once the lens has been formed, it is removed from the mold and inspected to confirm that the desired axis angle was achieved.
Accordingly, up to now, measuring the axis angle of a toric lens required measuring the axis angle on the lens itself. However, this requires an extra step during production and increases cycle time. The increase in cycle time can compromise machine output in most high speed automated environments and further compromise the efficiency of the system. Thus, there is a need to reduce this inefficiency when measuring the axis angle of a toric lens.